Deep fat fryer with swivel filter element

ABSTRACT

Disclosed is a deep fat fryer with a filtration system to continuously filter liquid cooking fat circulated by a pump. A fryer filter with a filter element of material such as carbon-impregnated cloth is located in the bottom of a fry tank and is rotatable about a swivel joint between a horizontal operating position and a vertical servicing position. In the vertical position the upper end of the filter is above the surface of the fat in the fry tank and the filter element may readily be replaced without draining the tank and without risk of leakage of fat.

CROSS REFERENCE TO RELATED APPLICATION

Some of the subject matter disclosed herein is also disclosed incopending U.S. application Ser. No. 569,125, "High Efficiency Deep FatFryer", filed Jan. 9, 1984, in the name of James R. Hurley et al, andassigned to the assignee of record herein.

BACKGROUND OF THE INVENTION

This invention relates to apparatus for frying foods and particularly toimproved deep fat fryers wherein the fat is circulated and filtered.

Deep fat frying of foods such as potatoes, meats, fish and pastries isperformed by immersing the foods in a vessel containing heated cookingoil or fat. Of considerable importance in such fryers are their systemsfor heating the fat and for maintaining its temperature uniformity andcleanliness.

Heating systems of deep fat fryers are generally of three types. Someheaters comprise gas-fired burner tubes or electric elements immersed infat held within the fryer vessel. A second category of fryer heatersincludes systems which apply heat to fat in the fryer vessel by exposingthe vessel to combustion gases or the condensible vapors of a heat pipeworking fluid. A third type of heater includes a heat exchanger externalto the fryer vessel through which fat is circulated, sometimes afterpassage through a filter.

Present deep fat fryers have one or more drawbacks or undesirableaspects. For example, commercially available fryers have relatively lowefficiencies and, if gas-fired, have high flue losses. Typicalcalorimetric efficiencies for available gas-fired fryers are in therange of 40-60 percent.

Prior art fryers which circulate fat for external heating may utilizesubstantial energy in pumping the fat, particularly if a filter isincluded in the circulation system. Replacement or cleaning of filtersin such units can be difficult, messy, and time-consuming, and cloggingor blockage of the filters requires an immediate shutdown of the fryer.

It is a primary object of the invention to provide a deep fat fryer withan improved filtration system.

It is an object of the invention to provide a deep fat fryer withfiltration whose filter minimizes risk of leakage of cooking fat duringoperation of the fryer and changing of the filter element.

It is also an object of the invention to provide a deep fat fryer whosefilter element may be easily changed without the use of shutoff valvesor the need to drain the fryer tank.

SUMMARY OF THE INVENTION

The invention concerns a deep fat fryer with an improved filtrationsystem in the bottom of a fry tank. The filter provides continuousfiltration of fat returning to the tank through an external loop such asone of two circulation loops arranged in parallel between external pumpmeans and a reservoir of fat in the tank. A swivel joint permitsrotation of the filter between a horizontal operating position and agenerally vertical position in which the upper end of the filter isabove the surface of fat. This facilitates replacement of a filterelement without drainage of the tank or use of shutoff valves, andwithout risk of leakage of fat.

A preferred fryer filter includes a perforated casing of generallyflattened cylindrical shape and a filter element of carbon-impregnatedcloth or other cloth or filtration medium mounted adjacent to the innersurface of the casing. Opposed ends of the filter casing are sealedaround a pipe through one end of which fat is delivered to the filterfrom a pump positioned below the tank. The other end of the pipe isclosed and fitted with a fastener such as a nut which may be removed inorder to replace the filter element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view in perspective of a deep fat fryer according to theinvention.

FIG. 2 is a front end view, partly broken away, of the fryer of FIG. 1.

FIG. 3 is a top view of the fryer of FIG. 1.

FIG. 4 is a side view, partly in section, of a preferred fryer such asthe fryer of FIG. 1 having a helical-shaped heat exchanger.

FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 4.

FIG. 6 is a top view of the helical-shaped heat exchanger of the fryerof FIG. 4.

FIG. 7 is a fragmentary side view, similar to that of FIG. 4, butillustrating a fryer having a serpentine heat exchanger.

FIG. 8 is a cross-sectional view taken along the line 8--8 of FIG. 7.

FIG. 9 is a sectional view taken along the line 9--9 of FIG. 8.

FIG. 10 is a top view, partly broken away, of a fryer according to theinvention, illustrating a preferred filter.

FIG. 11 is a side view of the embodiment shown in FIG. 10.

FIG. 12 is an end view of the embodiment shown in FIG. 10 showing thefilter both in an operating position and (in broken line form) in anupright position facilitating changing of the filter element.

FIG. 13 is a schematic of a control system for the fryer according tothe invention.

FIG. 14 is a graph with plots of test results showing the effect on fatquality of filtration of fat in the fryer.

DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred deep fat fryer illustrated in FIGS. 1-5 includes a vessel orfry tank 20 housed within an open-topped cabinet 22. The tank 20 isadapted to hold a cooking oil or other cooking liquid (hereafterreferred to as "fat") up to a level of approximately the lower edge ofan inclined shelf 26 and to also accommodate a wire basket or other foodholder for immersion into fat contained within the tank 20. Portions oftwo typical baskets 28 are shown in FIGS. 3 and 4. A crumb tray or grate30 divides the fat-holding portion of the tank 20 into a bottom zone 34and an upper, or fry, zone 38 which is unobstructed and of sufficientdepth for immersion of food held by the basket. The bottom zone 34carries a filter 40 through which a portion of the fat is continuouslycirculated as described later in this specification.

Outside of, and preferably below, the fry tank 20 are a heat exchanger44 and a burner 50 which are used to maintain the temperature of the fatat a desired level such as about 350° F. A pump 54 is connected to thefat-holding portion of the tank 20, for example, by mounting its inlet56 to the bottom wall 60 of the tank, and is operable to withdraw fatfrom the tank for heating and filtration. Between the pump outlet 64 andthe tank are two return loops each adapted to return part of the outflowof fat from the pump to the tank. One loop comprises a return pipe 66connected to the inlet of the filter 40 in the bottom zone 34 of thetank. The other return loop is adapted to direct flow to the heatexchanger 44 and includes a pipe 68 to carry fat from the heat exchangeroutlet 70 back to the tank 20. This parallel flow arrangement permitscontinuous filtration of the fat at modest power requirements and avoidsthe need for immediate shutdown of the fryer in the event of clogging ofthe filter.

As is shown in FIGS. 4 and 5, the pipe 68 which returns fat from theheat exchanger 44 to the fry tank 20 is connected to the rear wall 72 ofthe tank 20 at an upper, central location so as to discharge fat intothe fry zone 38 as turbulent jets directed towards the opposite (front)end of the tank 20. This directional flow and a high flow rate of fat,up to about ten gallons per minute for a fry tank having a workingvolume of about seven gallons or fifty pounds of fat, provides jettingaction and highly turbulent flow in the tank 20 which provides improvedcooking of foods such as French fries. One observed benefit of thejetting action or forced convection through the baskets has been moreuniform frying of foods held by the baskets. This permits, in fryers ofthe present invention, greater loading of the baskets--up to 7-8 poundsof fries per pair of baskets as compared to a typical loading of 3-4pounds for conventional fryers--without risk of incompletely-cookedfries in the interior or central portions of the baskets. Also, thejetting action permits cooking at a slightly reduced fat temperature,e.g. at about 10° F. lower than would otherwise be maintained. The lowertemperature prolongs useful life of the cooking fat and reducesconsumption of fuel.

A deflector 73 may be provided near the point of discharge of the pipe68 to prevent upward components of the turbulent jet flow from causingspattering or spraying of fat from the upper surface of fat held in thetank 20. Strong upward flow components, though not usually presentduring cooking operation, can occur if excess moisture becomes mixedwith fat circulated through the heat exchanger 44 and forms steam whichexpands rapidly when the mixture is jetted into the tank through thepipe 68. The deflector 73 (see FIGS. 3 and 4), which may be arectangular plate extending across a portion of the width of the frytank 20 and attached to an upper portion of the tank rear wall 72, actsas a guard to deflect flow downward and sideways away from the uppersurface of the fat. The deflector plate 73 also increases turbulence asthe main flow of fat is deflected sideways and downward around its edgesfor passage through the two adjacent fryer baskets 28.

The heat exchanger 44, which is formed in the shape of a helix in theembodiment shown in FIGS. 4-6, is mounted between the tank bottom wall60 and the burner 50 positioned near the base 74 of the cabinet 22. Withthis arrangement, upwardly directed combustion products from the burner50 pass into contact with outer surfaces of the heat exchanger 44 toheat fat being circulated therethrough. The combustion products alsotransfer heat by contact with the bottom wall 60 and the rear wall 72 ofthe fry tank 20 as these gases flow towards the exhaust 90 located inthe upper rear portion of the fryer.

A preferred heat exchanger 44 is a coiled, helical-shaped tube whosecentral axis 92 is substantially vertical. The radii of the heatexchanger coils 94 relative to the axis 92 decrease with increasingaxial distance from the burner 50 as shown in FIG. 4 in order to exposea maximum amount of surface of the heat exchanger 44 to the combustionproducts yet permit upward flow of combustion products between itscoils. To further promote heat transfer, external fins such asring-shaped fins 98 may be brazed or otherwise bonded to, or held ingood thermal contact with, the outside of the heat exchanger 44 alongall or a portion of its length. Suitable material for the fins 98 is alow carbon steel, and a preferred material for the tube forming the heatexchanger coils 94 is stainless steel.

The burner 50 may be a gas-fired lanced-port burner having one or morepipes 100 (for example, four are shown in FIG. 5) each connected to amanifold 102 and having a row of perforations along its upper portionfacing the heat exchanger 44. Atmospheric lanced-port burners arepreferred because of their simplicity, low cost, and quiet operation.The lanced-port burner 50 does not need a blower to provide air forcombustion but instead entrains its combustion air. Also, the burner 50is compatible with a system which does not require a forced draft fan toremove flue gases from the fryer. A gas pilot 104 is suitable forignition of the burner 50; alternatively, a spark igniter may be used.

Withdrawal and circulation of the fat external to the tank isaccomplished by means of the pump 54 which may be a centrifugal pumpwhose inlet 56 is bolted to the tank bottom wall 60. A preferred pump(FIG. 4) has a magnetically coupled motor 108 and an impellor (notshown). This permits separation of these components by a heat wall 110so that although the impellor housing 114 is contacted by combustionproducts of the burner 50, the pump motor 108 is not exposed to thesehigh temperature gases.

FIGS. 4, 10-12 show a preferred filter 40 for removing fat breakdownproducts (chiefly fatty acids) and particulate matter from the fatduring its circulation. Filtration increases the useful life of the fat,saving costs associated with replacement of the fat, including loss offryer operating time.

The filter 40, which is positioned in the bottom zone 34 of the tank 20,may (as illustrated herein) have a generally flattened cylindricalshape. At one end an inlet 116 is provided to receive fat from a conduit120. The conduit 120 in turn communicates with the return pipe 66through a swivel fitting 122 which extends through the front wall 124 ofthe fry tank 20.

As is best shown in FIGS. 10-12, the filter 40 comprises a casing 130with a plurality of perforations 131 through which fat is discharged tothe bottom zone 34 of the fry tank. A centrally-located pipe 132 runsalong the length of the filter 40 and includes an open end 134 near thefilter inlet 116 and a closed end 136 opposite the end 134. The pipealso has at least one discharge opening 138 for permitting flow of fatout of the pipe 132 to the space between the pipe 132 and a filterelement 140 positioned adjacent to the inner wall of the casing 130.Seals 142 and 144 surround the pipe 132 at locations of the opposed pipeends 134 and 136, respectively, at which these pipe ends extend throughopenings in the casing 130. The closed pipe end 136 has a threadedportion engaged by a lock nut 146 and a washer 148. The nut and washerbear against the seal 144 to hold the casing 130 in position against anelbow 150 at the filter inlet 116.

Any of several materials may be utilized in the filter element 140;candidate materials must, however, be suitable for operation at thetemperature of the circulating liquid fat (about 350° F.) and be capableof cleaning the fat without excessive pressure drop or tendency toescape from the filter. One preferred filter element 140 is acarbon-impregnated cloth which can be precut to size and used as adisposable, easily-installable cartridge. In some applications, whereexcessive discoloration of the fat is the major determinant of fat life,a material such as cotton paper may be a sufficiently effective,low-cost alternative for use as the filter element.

The filter element may be a free-standing cartridge or, as indicated inFIG. 11, the filter may include a screen 151 to hold the filter element140 in position adjacent to or against the inner wall of the casing 130.The screen 151, which is preferably attached to a casing cover 154, mayfacilitate replacement of certain filter elements and improve overallremoval of particulate matter and fat breakdown products from thecirculating fat.

The arrangement of the filter 40 and its location in the bottom zone 34of the fry tank 20 allow use of a tank of standard size and shape and ofsimple plumbing connections. Moreover, they permit changing of thefilter element 140 without risk of leakage of fat and without drainingfat from the tank 20. As is best shown in FIGS. 10 and 12, the filter 40and its associated conduit 120 are rotatable about the swivel fitting122 from a generally horizontal (operating) position to a generallyvertical position wherein the end of the filter 40 containing the locknut 146 is above the normal level of fat in the tank 20. Thus to installa new filter element 140, the fryer is turned off and a simple hook (notshown) such as a standard hook supplied with the filter 40 is first usedto remove the crumb tray 30. The hook is then engaged with a loop 152attached to the end of the casing 130 opposite the filter inlet 116, andthe filter 40 is rotated to the upright position shown in dashed form inFIG. 12. Upon removal of the lock nut 146 and the washer 148, the casing130 and seals 142, 144 may be slid upward and out of contact with thepipe 132. The casing cover 154 and the used filter element may then beremoved, a fresh filter element installed, and the filter reassembledand repositioned for continued fryer operation.

During operation of the fryer, a portion of the fat circulated by thepump 54 (e.g. about ten percent by volume) is directed through the pipe66 and the conduit 120 into the filter pipe 132. This fat emerges fromthe pipe opening 138 and then is forced through the filter element 140and out of the casing perforations 131 into the bottom zone 34 of thefry tank 20.

The fraction of circulated fat which passes through the filter 40 isgoverned by the effective resistances in the two return loops, which aredetermined by the relative sizes and lengths of the pipes 66, 68, and120 and the heat exchanger coils 94 as well as the flow resistance ofthe filter 40. In one fryer constructed and tested according to theprinciples disclosed herein about ninety percent of the circulating fatflowed through the heat exchanger 44 and the remaining ten percentflowed through the filter 40.

The parallel flow arrangement of the filter 40 and the heat exchanger44, unlike a series arrangement of these components, permits the flowfractions to be varied as required to obtain desired amounts offiltration and heating. Moreover, as mentioned previously, thisarrangement permits all or nearly all of the fat withdrawn from the tank20 to be circulated through the heat exchanger 44 in the event ofclogging of the filter 20. This allows normal fryer operation tocontinue until the time of a normally scheduled shutdown or until lossof filtration leads to fat quality which is unacceptable for furthercooking.

Under certain frying conditions, such as immediately following immersionof food of high moisture content into the fat held by the fry tank 20,the pump 54 may entrain substantial amounts of water or steam as itwithdraws fat from the tank. Unless measures are taken to avoid oraccommodate this moisture, vapor lock may occur within the pump 54,reducing the flow of fat through the heat exchanger 44 and the filter 40to unacceptably low levels. Entrainment of foam may also be a cause ofvapor lock.

One method of accommodating a vapor lock is to restart or recycle thepump after an automatic shutdown is effected by means of a "low flow"control system as described hereinafter. Alternatively, this vapor lockcondition may be avoided by limiting the batch size of any high watercontent foods cooked in the fryer or PG,16 temporarily reducing pumpspeed during their initial cooking when release of moisture is greatest.Entrainment of water may be prevented or reduced by the use of baskets28 with a solid bottom or plate which blocks release of moisture fromthe bottom of the baskets into fat below the baskets. Moisture ispermitted to flow out of the sides and top of the baskets, areas fromwhich moisture may more easily bubble upward and escape from the fryeras steam.

Use of a properly-designed crumb tray 30 in the fry tank 20 has alsobeen found to help prevent vapor lock of the pump 34 by inhibitingmigration of moisture from the upper zone 38 of the tank to the bottomzone 34. A crumb tray made from perforated sheet metal of fine mesh andlow percentage open area is preferred.

Moisture entrainment may be further inhibited by structures such as thestandpipe 160 and the baffle assembly 162 illustrated in FIG. 11. Thestandpipe 160 blocks entry into the pump 54 of water which may havesettled to the bottom of the tank due to its higher density relative tothe fat. The cage or baffle assembly 162 enclosing the standpipe 160helps to shield the standpipe 160 from direct intake of water releasedby food into the fry zone 38. It is important, however, that the "cage"or baffle assembly 162, if used, have sufficient open area so that, asparticles from food being fried accumulate on it, the pressure dropacross its inlet openings 164 remains low enough not to impair operationof the pump 54. The combination of the baffle assembly 162 and thestandpipe 160 provide an opportunity for moisture to vaporize and escapeupward through the fry zone 38 as bubbles or to collect as water at thebottom of the tank 20.

Control of the fryer is achieved by a system having pressure andtemperature sensors which are electrically linked to the burner 50 andthe pump 54. An electrical schematic of the fryer control system isgiven in FIG. 13, and FIGS. 2 and 4 show the locations of major elementsused in control of the burner and the pump. Included in the system is acook thermostat 170 which senses temperature of fat in the tank 20 andmaintains this temperature at a desired level by regulating a gas valve172 in a gas line 174 connected to the burner 50. One or more additionalthermostats may be provided such as a limit thermostat 175 (FIG. 2) toshut down the burner 50 and the pump 54 if the temperature of the fat inthe tank 20 exceeds a predetermined level such as 380° F., and a "cleanmode" thermostat 176 which senses and regulates temperature of a fluidsuch as a water-based cleaning solution which may be circulated throughthe fryer during a cleaning cycle. As shown in FIG. 13, the "cook" and"clean" modes of operation may be selected by appropriate positioning ofswitches 177 and 178.

In addition to the above-described thermostatic controls, a simplearrangement of two sensors permits operation of the burner 50 only whenfat flow through the pump 54 is equal to or greater than a prescribedlevel. This "low flow" control function is desirable to preventoverheating and rapid breakdown of the fat which would result fromcontinued burner operation at low flow rates. Such low flow conditionscan exist during normal start-up of the pump 54, or if a vapor lockoccurs in the pump, or if the pump motor fails, or if the heat exchanger44, the pump 54, or their plumbing becomes blocked.

One preferred "low flow" control comprises two pressure switches 180 and182 (FIGS. 2 and 13) positioned in a single tap 184 at the pump outlet64 and operable to detect increases or decreases in the static pressureof fat circulated by the pump 54. Two pressure switches are neededbecause the pressure changes due to clogging or a plumbing blockagedownstream of the pressure sensing location are opposite to those whichoccur if the pump motor 108 fails or the pump 54 suffers a vapor lock orthe pump inlet becomes clogged. As is illustrated schematically in FIG.13, the pressure switches 180 and 182 are electrically connected inseries between a power source 186 and the gas valve 172 such that bothswitches must be closed in order for the gas valve 172 to be open fordelivery of fuel gas to the burner 50. Normally open switch 180 is setto close and remain closed only if the static pressure it senses isequal to or greater than a value corresponding to an acceptable flowrate of fat. A vapor lock, failure of the pump motor 108, or clogging ofthe pump inlet 56 or the sensor line to the switch 180 will act to openthe switch 180 and shut off or prevent flow of gas through the valve172. Normally closed switch 182 remains closed unless the staticpressure it senses is equal to or greater than a predetermined value dueto a clog in the fat circulation system beyond the pump outlet 64. Itshould be noted that if the normal flow rate of fat through the heatexchanger 44 is substantially greater than through the filter 40,clogging of the filter 40 will not produce an unacceptably low flow rateof fat so as to open the switch 182. Thus fryer operation may continuewith partial or no filtration until the time of a scheduled shutdown.

An alternative "low flow" control includes a temperature sensor 188 inthe exhaust (FIG. 4) instead of the second pressure switch 182. The useof a temperature sensor 188 in place of, or in addition to, the switch182 may be preferable to two pressure switches since it is possible,were downstream clogging accompanied by blockage in the sensor line tothe switch 182, that the switch 182 would not sense downstream cloggingso as to deenergize the gas valve 172. (The switch 180, on the otherhand, will failsafe if blocked--i.e., act to terminate gas flow.)

The fryer control system also includes a pump cycling arrangementoperable in conjunction with the "low flow" control and which is used toovercome vapor locks by "burping" bubbles of vapor from the pump 54. Asindicated in FIG. 13, this arrangement includes a vapor lock timer 190connected between the pump motor 108 and the pressure switch 180. In theevent of a "low flow" event sensed by the pressure switch 180, theswitch 180 moves from a closed (C) position to an open (O) position,closing the gas valve 172. After a brief time interval, a timer switch191 opens, turning off the pump motor 108 for a selected interval oftime such as ten seconds. This releases suction at the pump inlet 56 sothat vapor trapped in the pump 54 may escape into the fry tank 20,unblocking the pump. At the end of the time delay, the switch 191restarts the pump motor 108 for a selected interval such as threeseconds. If the vapor lock or other malfunction has been eliminated,pressure at the pump outlet will then increase and switches 180 and 182will permit gas to flow through the valve 172 for restart of the burner50 and continued fryer operation. If the pump restart is unsuccessful,the timer 190 will initiate successive "burp" cycles including pumpshutdowns and attempted restarts until fryer operation can be continuedor other measures are deemed necessary.

Also shown in FIG. 13 is circuitry for operating the fryer in a "rinse"mode and a "dry" mode. Use of a "dry" cycle is desirable to removeresidual water from the heat exchanger 44 and other plumbing afterdraining used fat from the fryer and cleaning the fryer. Removal of thiswater avoids vapor lock problems of the pump 54 which could occur duringfat heat-up or fryer operation were water to remain and thus mix withnew fat loaded into the fry tank 20.

The dry cycle is initiated by moving the switch 177 into the "rinse"position and depressing a momentary-closing switch 193, which activatesa timer 195. The timer 195 opens the gas valve 172 for a selected timeinterval such as two minutes, during which the burner 50 fires againstthe heat exchanger 44 to expel residual rinse water through a drainvalve 197.

Rinsing of the fryer, when desired, is accomplished by moving the switch177 to the "rinse" position, which provides power to the pump motor 108while excluding power from the rest of the control system by opening anormally-closed contact 199.

FIGS. 7-9 illustrate a fryer similar to those described herein up tothis point except that its heat exchanger 200 comprises a serpentinetube 202 instead of the helical or coil-shaped tube illustrated in FIGS.4-6. The tube 202 is configured as a single-row heat exchanger--i.e., itlies essentially in a plane parallel to the burner 50 and the bottomwall 60 of the fry tank 20. External fins 204 are preferably bonded to,or integral with, the outer surface of the tube 202 to promote heattransfer from the combustion products of the burner 50 to fat circulatedthrough tube heat exchanger tube 202.

In use and operation, the fryer is connected to a source of combustiblegas such as natural gas and to a source of electrical power, and the frytank 20 is filled with fat to a level about in line with the bottom ofthe inclined shelf 26. If desired, the liquid fat in the tank 20 may beobtained by melting a solid block of fat by cycling the burner 50 andthe pump 54 for short, regular intervals or by equipping the fryer withan electrical heating tape located beneath and in good thermal contactwith the bottom of the fry tank 20. After sufficient liquid fat isavailable, the pump 54 is started for a normal frying operation. When asufficient flow rate of fat is attained, the gas valve 172 opens and theburner 50 is lit. The fat in the tank circulates continuously throughthe heat exchanger 44 and the filter 40 and quickly attains a levelsuitable for frying of food.

Efficiency and filtration tests were conducted on commercially availablegas-fired fryers and on a fryer constructed in accordance with theinvention and having a heat exchanger of serpentine shape. Calorimetricefficiency, determined from the proportion of combustion energy impartedto the fry tank 20, was measured as 74-77 percent in three tests of thefryer of the invention. This is considerable higher than theefficiencies of 56-58 percent measured for the nearest competitor, and40-45 percent for three other fryers.

The benefit of filtration on fat quality is illustrated in FIG. 14,which shows plots of the concentration of titratable free fatty acids (astandard indicator of fat breakdown) measured in samples of fat taken atregular intervals during two tests in each of which about 400 pounds ofpotatoes (frozen and blanched French fries) were cooked. In one test,the aforementioned fryer of the invention was used and its filterelement was changed after each 100 pounds of French fries were cooked.In the other test no filter was used. The lower concentrations of acidsachieved with filtration suggest a distinct benefit of filtration, andit is believed that further performance improvements are obtainable byuse of a heavier filter element or a larger filter, or more frequentchanges of the filter element.

Tests were also conducted to determine production rates of a fryerconstructed in accordance with the invention. Results showed thatproduction rates of about 90 lbs/hour of French fries (starting withfrozen fries) were achievable at a natural gas input of about 80,000BTU/hour. This production rate is considered to be about fifty percenthigher than those attained by commercially available fryers.

Although the invention has been shown and described with reference tocertain preferred embodiments, it is apparent that the fryer may beembodied in other specific forms without departing from the spirit oressential characteristics of the invention. For example, the fryer maybe heated by immersion-type electrical heaters instead of by gas-firedburners; in this form only a single return loop without heat exchangerneed be included in the fryer. The scope of the invention is indicatedby the appended claims, and all changes which come within the meaningand range of equivalency of these claims are intended to be embracedtherein.

What is claimed is:
 1. In a deep fat fryer including a tank for holdingcooking fat, a filter for filtering the fat, and circulating means forwithdrawing fat from the tank and then returning fat to said tank, theimprovement wherein said filter is positioned in the bottom of said tankand includes an outer casing having a plurality of holes for dischargeof fat, a filter element within said casing, an inlet for receiving fatto be filtered, and lock means for permitting removal and replacement ofsaid filter element, said circulating means includes a return conduitextending through said tank and connected to the inlet of said filterfor delivering fat to the filter, said conduit having a swivel jointpermitting rotation of said filter from a generally horizontal positionto a generally vertical position, and said outer casing of the filterhaving a length such that upon rotation of the filter to a generallyvertical position a portion of said filter including said lock meansextends above the normal surface level of said fat.
 2. A deep fat fryeras in claim 1 wherein said filter element comprises a carbon-impregnatedcloth.
 3. A deep fat fryer as in claim 2 wherein said filter element ismounted on a screen adjacent to the inner surface of said outer casingof the filter.
 4. A deep fat fryer as in claim 1 wherein said outercasing has a generally flattened, cylindrical shape and an opening ineach of its opposed ends, and said filter further comprises:a piperunning the length of said casing and extending through said openings inthe opposed ends of the outer casing, said pipe including an open endconnected to said return conduit, a closed end extending through theopening in the end of said casing opposite the inlet of said filter, andat least one outlet in the portion of the pipe within said casing foroutflow of fat from said pipe; and a seal around said pipe at saidopenings in the opposed ends of the outer casing.
 5. A deep fat fryer asin claim 4 wherein said lock means includes a fastener removablyattached to the closed end of said pipe adjacent to the end of thecasing opposite the inlet of said filter; and said seals around the pipeare slidable with said casing along the pipe upon removal of saidfastener.